Increase in the use of sophisticated car stereos in recent years has caused an increase of thefts of the stereos. In order to prevent the theft of the stereo, there has been many proposals of antitheft systems.
In accordance with one of the systems, a car stereo is provided with means for registering an identification code. The user stores an arbitrary code in a memory provided in the car stereo so that the car stereo cannot be operated unless the correct code is input thereto. Such a car stereo is shown in FIG. 4. Namely, disposed on a front panel 2 of a car stereo body 1, are a cassette opening 3, a plurality of push buttons 4 for operating a cassette tape, and a liquid crystal display panel 9 for indicating information such as current time and a received frequency. At a lower portion of the opening 3, preset switches 5 are provided for setting a frequency of for example, FM channel and for displaying the set frequency. A plurality of function switches 6, time adjusting switches 7 and a display exchange switch 8 are provided at a lower portion of the preset switches 5. A volume control knob 10 and a matrix of code keys 11 are disposed at the left side of the front panel 2.
As shown in FIG. 5, the control system of the car stereo has a microcomputer 13 applied with a voltage of 5 V from a 5 V supply circuit 12 and connected to the liquid crystal display panel 9 through a display driver 17. An 8 V supply circuit 14 is connected to a lighting lamp 15 through a protection circuit 16 and to other mechanisms such as those for playing a cassette tape. The protection circuit 16 operates to stop the supply of power when the microcomputer 13 detects a problem such as a short-circuit.
The control system further has a key code producing circuit 18 which transmits an input code in accordance with the operation of the code keys 11. The code is fed to the microcomputer 13. A memory 19 is provided for storing the code set by the user.
Prior to playing the car stereo, the user registers a code of his choice by operating the code keys 11, which is then stored in the memory 19. In order to play the car stereo, the user inputs the registered code by operating the code keys 11. The code is applied to the microcomputer 13 which compares it with the stored code in the memory 19. When the codes coincide with each other, the protection circuit 16 is released so as to apply the 8 V to actuate the liquid crystal display panel and to operate other mechanisms of the car stereo. If an incorrect code is input, the protection circuit 16 stays operative so that the car stereo cannot be played, thereby preventing theft of the car stereo.
The control system must be provided with the key code generating circuit so that the construction thereof becomes complicated. Moreover, a thief might succeed in detecting the set code through numerous attempts at the keys.
Another type of a car stereo is built into a quick-removal chassis in a dashboard. Referring to FIG. 6 showing such a car stereo, there are provided various pushbuttons, keys, knob and cassette tape opening, which are identified by the same references as the car stereo shown in FIG. 4. In addition, the car stereo is provided with a rotatable handle 27 and a connector (not shown) provided at the back of the body 1. When the car stereo body 1 is mounted in the dashboard of an automobile the connector of the car stereo is connected with a connector provided in the dashboard, thereby supplying power to the car stereo from the automobile.
When leaving the automobile, the driver pulls out the car stereo body 1 from the dashboard with the handle 27 and takes it with him or her. However, the car stereo is heavy, hence undesirable to carry about.
Referring to FIG. 7, there has further been proposed a car stereo having a body 1a and a detachable grille 20 attached to the body 1a. The same references as those in FIGS. 4 and 6 designate the same parts.
A front panel 1b having a recess 1c is fixed on the outer periphery of the front portion of the car stereo body 1a. The recess 1c is so dimensioned that the grille 20 snugly fits therein. At an upper central portion of the recess 1c is formed a cassette opening 21. Three push buttons 22 are provided on the opposite sides of the cassette opening 21. A connector 23 which is connected to a connector (not shown) provided at the 20 back of the grille 20 when the grille is attached, is formed on the panel 1b.
The grille 20 has the cassette opening 3 which coincides with the cassette opening 21 of the body 1a and push buttons 4 provided opposite sides of the 25 cassette opening 3 corresponding to the push buttons 22 of the body 1a for operating a cassette tape. At a lower portion of the opening 3, preset switches 5 are provided. The volume control knob 10, function switches 6, time adjusting switches 7 and a display exchange switch 8 are provided at a lower portion of the preset switches 5. The liquid crystal display panel 9 is disposed at a right portion of the grille 20.
In order to attach the grille 20 on the body 1a, a groove 24 formed in one side of the grille 20 is engaged with a projection (not shown) projecting into the recess of the panel 1b from an inner edge thereof, thereby positioning the grille 20. The grille 20 is then pivoted about the projection so as to close in on the panel 1b, so that a push-push mechanism (not shown) provided in the body 1a engages with a recess (not shown) formed at the inner corner of the other end of the grille 20.
In order to detach the grille 20 from the panel 1b, the grille 20 at the side having the inner recess is depressed. The push-push mechanism is accordingly operated to project the grille 20. The grille 20 is pivoted about the projection and pulled off the panel 1b of the body 1a.
In order to operate the car stereo, each of the stereo body la and the grille 20 has a control system as shown in FIG. 8. The control system for the grille 20 comprises a microcomputer 26, key code producing circuit 18 connected with the operation switches 5 to 8 so as to transmit a key code in accordance with the operation of the switches, display driver 17 operated by the microcomputer 26 for driving the liquid crystal display panel 9, lighting lamp 15, and the connector 25. The control system for the stereo body 1a comprises the microcomputer 13 for controlling various operation modes of the car stereo, 5 V supply circuit 12 for supplying the voltage of 5 V to the microcomputer 13, 8 V supply circuit 14, protection circuit 16, and the connector 23.
The connector 23 of the body has a 5 V supply terminal 23a, data output terminal 23b, data input terminal 23c, key code input terminal 23d and 8 V supply terminal 23e. The terminals 23a and 23e are connected to the 5 V supply circuit 12 and the 8 V supply circuit 14, respectively, through the protection circuit 16. The data output and input terminals 23b and 23c are connected to the microcomputer 13. The microcomputer 13 is connected to the protection circuit 16 so as to operate the circuit. When the grille 20 is detached, the protection circuit 16 stops the supply of voltage from the 5 V supply circuit 12 and 8 V supply circuit 14 to the terminals 23a and 23e. Thus, the car stereo circuitry provided in the body 1a is protected from damage which is caused by the short-circuiting of the exposed terminals 23a to 23e.
The connector 25 of the grille 20 has terminals which correspond to the terminals 23a to 23e of the connector 23. Namely, there is provided a 5 V input terminal 23a, data input terminal 23b, data output terminal 23c, key code output terminal 23d, and 8 V input terminal 25e. The terminal 25a is connected to the microcomputer 26 so as to supply the 5 V voltage thereto. The microcomputer 26 receives data from the microcomputer 13 through the data input and output terminals 25b and 23b dependent on the operation of the operation switches 5 to 8 on the grille 20. A signal of the microcomputer 26 is applied to the microcomputer 13 through data output terminal 25c and data input terminal 23c. The microcomputer 26 applies a control signal to the display driver 17 to indicate the received data on the display 9. The 8 V input terminal 25e is connected to the lamp 15 for lighting the display 9.
When the grille 20 is taken out, the stereo body 1a loses the appearance of a car stereo. Hence, if the driver takes the grille 20 with him or here when leaving the vehicle, the theft of the car stereo is prevented.
However, the car stereo does not completely prevent theft, since the existence of the car stereo may be found despite its appearance. The stolen car stereo body without the grille may be easily played if an appropriate grille is stolen elsewhere and attached to the body.
Thus, none of the conventional car stereos are effective in preventing theft.